BACKGROUND:The Dark Star was an artifact sealed deep in Toad Town Caves. He was once unleashed and started wreaking havoc. This is why he was sealed. After a long time Fawful freed the Dark Star. The Dark Star turned him into Dark Fawful, then it proceeded to steal DNA from Bowser's body.
The Mario bros. succeeded in removing the Dark Star from Bowser's body, but it managed to transform into Dark Bowser after absorbing the Dark Fawful Bug. The Dark Star was the weak spot for the Dark Fawful Bug.